Chapter 2
Voltage, Current,
and Resistance
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C.Y. Lee
Objectives
Describe the basic structure of an atom
Explain the concept of electrical charge
Define voltage and discuss its characteristics
Define current and discuss its characteristics
Define resistance and discuss its characteristics
Describe a basic electric circuit
Make basic circuit measurements
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Atomic Structure
An atom is the smallest particle of an element that
retains the characteristics of that element
An atom has a nucleus,
consisting of positively
charged particles called
protons, and
uncharged particles
called neutrons
The basic particles of
negative charge, called
electrons, orbit the
nucleus
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Electron shells and Orbits
Electrons orbit the nucleus at discrete distances
from the nucleus
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Electron shells and Orbits
Orbits are grouped onto energy bands known as shells
A given atom has a fixed number of shells
Each shell has a
fixed maximum
number of
electrons (2N2)
permissible at
energy levels
(orbits)
The copper atom
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Valence Electrons
Electrons with the highest energy exist in the outermost
shell, known as the valence shell, and electrons in this
shell are called valence electrons
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Valence Electrons
If a valence electron acquires enough external energy to
leave the atom, the process is known as ionization
The escaped electron is called a free electron
0
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Categories of Materials
Conductors readily permit current flow, due to a
large number of free electrons in the material
– Conductors are characterized by 1, 2, or 3 valence
electrons in their atomic structure
Semiconductors have 4 valence electrons
Insulators have few free electrons
– Insulators are characterized by more than 4 valence
electrons in their atomic structure
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Energy Band Diagrams
or
Insulator
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Semiconductor
9
Conductor
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Electrical Charge
The charge of an electron and that of a proton
are equal in magnitude but opposite in polarity
The force acting between charges is called an
electric field
Coulomb’s law:
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Q1Q2
F = k 2 (N)
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Coulomb
A single electron has a charge of 1.6 × 10-19 C
One coulomb is the total charge possessed
by 6.25 × 1018 electrons
The total charge Q, for a given number of electrons
Q = (number of electrons) × (1.6 × 10-19) C
Example:
How many coulombs of charge do 93.8 × 1016 electrons
represent?
Q = (93.8 × 1016) × (1.6 × 10-19) C = 15 × 10-2 C = 0.15 C
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Formation of positive and
negative ions
(Positive Ion)
H+
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(Negative Ion)
Cl−
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Voltage
The unit of voltage is the volt (V)
One volt is the potential difference (voltage) between
two points when one joule of energy is used to move
one coulomb of charge from one point to the other
W
V=
(V)
Q
Example:
How much energy is used to move 50 C from one point in a
circuit to another when the voltage between the two points
is 12 V?
W = Q × V = 50 × 12 J = 600 J
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Batteries
A battery is a type of voltage source that converts
chemical energy into electrical energy
The way cells are connected, and the type of cells,
determines the voltage and capacity of a battery
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Other Voltage Sources
Solar Cells convert light energy into electrical energy
+
−
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Other Voltage Sources
Generators convert mechanical energy into electrical
energy
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Other Voltage Sources
Electronic power supplies do not produce electrical
energy, but they convert the ac voltage from the wall
outlet into a constant dc voltage for use in our circuits
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Current
The movement of free electrons from negative to
positive is electrical current (I)
I
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Current
electrical current is the rate of flow of charge
Q
(A)
I=
t
Example:
Ten coulombs of charge flow past a given point in a wire
in 2 s. What is the current in amperes?
I = 10/2 A = 5 A
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Resistance
The property of a material that restricts the flow of
electrons is called resistance (R)
Where there is current through any material that
has resistance, heat is produced by the collisions
of electrons and atoms
One ohm of resistance exists if there is one
ampere of current in a material when one volt is
applied across the material
I
V
(Ω)
Ohm’s law:
R=
+
−
I
V
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Resistivity
l
(Ω) ,
R=ρ
A
ρ : Resistivity (Ω⋅cm)
l
A
身體各部份的電阻係數(以雙手間為100%)
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Resistivity
生物組織器官的電阻係數
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Resistivity
l2
l2
l
=ρ
R= ρ = ρ
A× l
V
A
l
V1
A
ΔV
V2
R02
ΔR ≈
2 ΔV
ρl
(V1 ≈ V2 = V )
Example:
若手臂在心臟舒張時的阻抗是50Ω，在心臟收縮時阻抗改變
2Ω，如果手臂的長度是10cm，手臂的電阻係數是1.5Ωm，
則手臂在心臟收縮時體積改變量是多少?
ρl 2
150 ×10 2
3
cm
ΔV = 2 ΔR =
×
2
=
12
R0
50 2
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Conductance
Conductance (G) is the reciprocal of resistance:
1
(S)
G=
R
The unit of conductance is siemens (S)
Example:
What is the conductance for a 22 kΩ resistor?
G = 1/(22 kΩ) = 45.5 μS
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Conductance
σ=
1
ρ
Example:
(S/cm) , σ : Conductivity
部分金屬與人體組織(組織液)之導電係數(S/m)
相同電壓加在尺寸大小與形狀均相同的銀棒與肌肉時，通過
的電流相差多少倍？
I
σ
61×106
銀
I肌
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=
25
銀
σ肌
=
0.4
= 1.5 ×108
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Resistors
Carbon-composition resistor
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Resistors
Surface mount resistor is available as small resistor
chip
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Resistors
Film resistors
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Resistors
Wirewound resistors are used where high power
ratings are required
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Color-code bands on a resistor
1st band is the first digit of the resistance value
2nd band is the second digit of the resistance value
3rd band is the multiplier (number of zeros)
4th band indicates the tolerance
R = 82000 Ω ± 5%
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Resistor 4-band color code
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Precision Resistors
Precision resistor
R = 8230000 Ω ± 2%
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Precision Resistors
Example:
Find the resistance value in ohms and the percent tolerance?
R = 327 Ω ± 0.5%
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Alphanumeric Labeling
Two or three digits, and one of the letters R, K, or
M are used to identify a resistance value
The letter is used to indicate the multiplier, and its
position is used to indicate decimal point position
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Variable Resistors
Variable resistors are designed so that their
resistance values can be changed with manual or
automatic adjustment
≡
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Variable Resistors
Two types of automatically variable resistors
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The Basic Circuit
A closed circuit is one in which the current has a
complete path
An open circuit is one in which the current path is
broken, or incomplete
e−
e−
(a)
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Ground
Ground is the reference
point in electric circuits
and has a potential of 0 V
with respect to other points
in the circuit
All ground points in a
circuit are electrically the
same and are therefore
common points
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Basic Circuit Measurements
A voltmeter measures voltage across (in parallel)
a resistance or load
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Basic Circuit Measurements
An ammeter is inserted in the current path (in
series) to measure current
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Basic Circuit Measurements
Resistance is measured across a resistor, out-ofcircuit, with an ohmmeter
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Electric Shock
Shock hazard in terms of three basic current path groups
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Electric Shock
Physical effects of electrical current
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Summary
An atom is the smallest particle of an element that
retains the characteristics of that element
Free electrons make current possible
Like charges repel, opposite charges attract
A single electron has a charge of 1.6 × 10-19 C
One volt is the potential difference between two
points when one joule of energy is used to move
one coulomb from one point to the other
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Summary
One ampere is the amount of current that exists
when one coulomb of charge moves through a
given cross-sectional area in one second
Resistance limits current
One ohm is the resistance when there is one
ampere of current in a material with one volt
applied across the material
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Summary
An electric circuit consists of a source, a load, and
a current path
An open circuit has an incomplete current path
A closed circuit has a complete current path
An ammeter is connected in line with the current
path
A voltmeter is connected across the current path
An ohmmeter is connected across a resistor (the
resistor must be out-of-circuit)
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